The present invention relates to the field of rare-earth doped fiber amplifiers and, more particularly, to an efficient use of residual pump power within the amplifier arrangement.
In the last decade, rare-earth doped fiber amplifiers in general, and erbium doped fiber amplifiers (EDFAs) in particular, have been extensively used in optical telecommunication systems as a means to amplify weak optical signals between telecommunications links, particularly signals at or near the 1550 nm wavelength. Various designs of these amplifiers have been proposed to provide efficient performance, where efficiency is measured in terms of high optical gain, low noise figure, high output power and pump efficiency. Recently, with the use of EDFAs in new applications such as multiple WDM systems and analog CATV systems, high optical power (along with low noise) has become essential in order to overcome the splitting losses and to have relatively high optical powers at receivers. High power levels can be achieved by increasing the pump power near the 980 nm or 1480 nm wavelengths. However, the semiconductor lasers conventionally used to emit at these wavelengths are problematic in terms of power scalability and overall lifetime.
As an alternative to providing an increased power for these newer applications, co-doping of the fiber amplifier has been proposed, where in most cases a co-doping of Er+3 and Yb+3 is used. Such a co-doping increases the amount of pump absorption and offers a flexibility in selection of the pump wavelength, owing to the broad absorption band of Yb+3 (from 800 to 1100 nm) in glass. In glass fibers which contain phosphorus, ytterbium can absorb pump power available from diode-pumped Yb or Nd-doped laser sources near 1064 nm and efficiently transfer the energy to erbium ions for power application near 1550 nm. To date, several fiber amplifiers with Er+3xe2x80x94Yb+3 co-doping that are pumped with a 1064 nm Yb or Nd-cladded pumping lasers have been demonstrated with co-, counter-, or side-pumping schemes.
In general, a conventional Er+3xe2x80x94Yb+3 doped fiber amplifier consists of a pump, multiplexer, Er+3xe2x80x94Yb+3 doped fiber and an isolator. FIG. 1 illustrates an exemplary prior art doped fiber amplifier 10, which is capable of producing optical power on the order of a few watts. As shown, amplifier 10 includes an input isolator 12 and output isolator 14, with a section of co-doped Er+3xe2x80x94Yb+3 fiber 16 disposed therebetween. A wavelength division multiplexer 18 is used to couple a pump source 20 (such as, for example, a diode-pumped laser emitting at 1064 nm) into fiber amplifier 16. An input optical signal Pin (at a wavelength of, for example, 1550 nm) is applied as an input through isolator 12 to doped fiber section 16. As shown in FIG. 1, the propagation direction of the pump signal from source 20 is opposite that of input signal Pin. Such an arrangement is commonly referred to as a xe2x80x9ccounter-pumpingxe2x80x9d amplifier. In a co-pumped amplifier, the pump signal and input signal both travel through the doped fiber in the same direction. In most cases, a counter-pumping arrangement is preferred since it provides a better optical efficiency (although co-pumping yields a lower noise figure). FIG. 2 contains a graph of efficiency (measured in terms of the ratio of the signal power to the pump), as a function of input signal power (dBm). Where the signal is high, the power can be extracted more efficiently. Therefore, the higher pump power is where the signal has been amplified. This improved efficiency can be attributed to the fact that Er+3 clusters in the fiber medium can be bleached more efficiently when the pump and signal propagate in opposite directions, resulting in more ions contributing to the Amplified. Spontaneous Emission (ASE), as well as improved efficiency.
For a given length of Er+3xe2x80x94Yb+3 doped fiber, the output power (Pout) increases linearly as a function of the injected pump power (for a constant input signal power), as shown in FIG. 3. This increase in output power continues even in deep compression of the gain medium by the high pump power. On the other hand, this increase of the pump power also results in high residual power at the end of the fiber. The residual pump power in an Er+3xe2x80x94Yb+3 doped fiber amplifier operating in a high saturation regime is quite high when compared to a conventional Er+3 doped fiber amplifier where excited state absorption (ESA) also extracts energy from the pump, reducing further optical efficiency as the 980 nm pump power increases. Therefore, in Er+3xe2x80x94Yb+3 fiber amplifiers, high residual pump power at 1064 nm is problematic, particularly for multistage amplifier designs, where this residual power may be harmful to other optical components in the system. Referring to FIG. 3(A), at an output power of 32 dBm, a residual pump power of 26 dBm remains (for an input pump power of 36 dBm), using a doped fiber section having a length of 11.5 m. Thus, a need remains in the art for providing the improved efficiencies offered by a co-doped Er+3xe2x80x94Yb+3 fiber amplifier, while reducing (or eliminating) the residual pump power associated with such an arrangement.
The need remaining in the prior art is addressed by the present invention, which relates to the field of rare-earth doped fiber amplifiers and, more particularly, to an amplifier arrangement including efficient re-use of residual pump power within the amplifier.
In accordance with the present invention, a fiber amplifier is formed to comprise at least two sections of rare-earth doped optical fiber. A pump source is coupled to one section of fiber, where the residual pump power at the exit of this section of fiber is thereafter redirected to a WDM (wavelength division multiplexer) which applies this residual pump as the pump signal input to the remaining co-doped section of fiber; the length of the fiber being determined based upon the amount of residual power present at the output of the second section. In the preferred embodiment, the pump signal is coupled to the second fiber section and the residual pump power is coupled into the first section.
In an alternative, multistage embodiment, a pump source is directly applied to the final stage of the amplifier (that is, to the final section of co-doped fiber), with the residual power applied as the pump input to the preceding stage. The residual power remaining after amplification at this stage is then similarly coupled into the prior stage, and continuing in a like manner to efficiently use all of the available pump power. In these arrangements, the length of fiber in each stage decreases from the final stage to the first stage. Multiple pump sources can also be used, with each residual pump applied as an input to another section.
Various embodiments of the present invention are possible, including co-pumped, counter-pumped and side pumping, and various combinations of the above, as described in detail in association with the following illustrations.